Here's why:

* Density is a measure of mass per unit volume.

* Alcohol (ethanol) has a lower density than water. This means that a given volume of alcohol will weigh less than the same volume of water.

* When you mix alcohol and water, the alcohol molecules fill in the gaps between the water molecules, making the solution more compact. This increased compactness means that a given volume of the mixture contains more mass than the same volume of pure water.

Therefore, a mixture of water and alcohol is actually denser than pure water, not lighter.

Why the confusion?

It's likely that you're thinking of the fact that alcoholic beverages float on top of water. This is due to the presence of other substances like sugars and carbonation, which further affect the density of the beverage. These substances can make the overall mixture less dense than water, causing it to float.

In summary, the density of a water-alcohol mixture is higher than pure water. The perception that it's lighter likely stems from the behavior of alcoholic beverages, which can float due to the presence of other components.